Metal-complexes can be designed in a molecular basis to show various properties, depending on the combination of the kinds of metals and the linking molecules. These materials have a potential in a wide range of applications, and thus it is expected to be applied in various fields such as drugs, luminescent materials, coating medium, etc. On the other hand, fine particles are made fine from bulk solids, and, for example, nanometer-size fine particles often drive a novel character. Such an appearance of the novel characters has extensively been studied and used. For example, semiconductor or metal fine particles show various properties depending on their particle sizes. A stained glass owes its multiple colors to metal ultrafine particles. In addition, the fine particles of a magnetic oxide such as ferrite have been already put into practical use in, for example, DNA analysis.
By the way, among the metal-complexes, Prussian blue and its analogues, i.e. Prussian blue-type metal complex, have extensively been researched and studied for applying them to the practical use. Prussian blue has used as a blue pigment from the past, and besides it has been studied in progress as a potential material for applications of advanced engineering fields such as displays and biosensors. FIG. 17 shows the crystal structure of the Prussian blue-type metal complex. The structure is relatively simple, and is such that two kinds of metal atoms (metal atoms 221 (MA) and metal atoms 224 (MB) in the figure) assembling NaCl-type lattices are three-dimensionally crosslinked with cyano groups (carbon atoms 222 and nitrogen atoms 223). As the metallic atoms of MA and MB, various metals other than an iron atom can be used. The character such as magnetism, electrochemistry, or optical responsiveness can be varied, by the substitution of the metallic atoms (referring to Patent Literature 1).
Downsizing of particles of the Prussian blue-type metal complex plays an important role in an attempt to put the metal complex into practical use. Such downsizing can improve the dispersibility of the metal complex in a solvent. As the dispersibility improves, an inexpensive and high quality film formation method can be employed, such as spin coating and fine processing using various printing methods. As a result, it can be expected the application including a biosensor, a display device, and the like is expanded. Such materials as described below have been reported as magnetic materials: a material that expresses its magnetism at room temperature; and a material the magnetism of which can be regulated by an external stimulus such as irradiation with light or the application of a pressure. The materials are assumed to find use in a wide variety of applications when the materials are turned into nanoparticles. Investigations have been conducted on the application of the magnetic nanoparticles of oxides to memory devices and biomaterials, and the Prussian blue-type metal complex is also assumed to find use widely in the same applications.
There have been a few proposals these years as to the method to produce Prussian blue-type metal complex. For stabilizing fine particles, it is effective to cover the fine particle with molecules. As the molecules for stabilizing ultrafine-particles, which has been practically employed so far, there are a surface active agent (used in an inverted micelle method) of AOT (di-2-ethylhexylsulfosuccinate sodium salt) or the like, a water soluble polymer of poly(vinylpyrrolidone) (PVP)(referring to Patent Literature 2) or the like, a protein of ferritin (referring to Non Patent Literature 1) or the like, and a relatively low molecular weight molecule containing an amino group like oleylamine (Patent Literatures 3,4).
As a method of fixing a Prussian blue-type metal complex on a substrate, electrolytic deposition by which a thin film is obtained has been generally employed so far (referring to, for example, Patent Literature 5). However, this method is not carried out by using fine particles. As a result, characteristic properties of the fine particles can not be used by the method. Further, the electrolytic deposition has such problems that microfabrication is difficult to be conducted and a large scale equipment is needed to produce a large number of films at the same time. In addition, a method of forming layers of Prussian blue-type metal complex particles in a layer-by-layer manner by holding a binder layer other than a metal complex between the layers of Prussian blue-type metal complex particles is disclosed (referring to Non Patent Literature 2). However, this method is extremely low efficient in production, and particularly formation of a thick film is difficult. In the case in which a water-soluble high-molecular compound binder is used as described in Patent Literature 2, water stability is not expected. Further, if a substantial amount of the binder component other than a metal complex is incorporated therein, properties of the Prussian blue-type metal complex such as electric response are affected thereby.